232 STUDIES ON FERMENTATION. 



assure ourselves that this organism has not taken the place 

 of the yeast we are endeavouring to purify. Growths at a 

 A'ery low temperature are of great help in enabling us 

 to get rid of all ferments that are foreign to " low " yeast, 

 and should be resorted to in all cases where this yeast is to 

 be purified. 



Another method of purification, which is perhaps quicker, 

 although inferior in other respects, consists in the employment 

 of carbolic acid — that is to say, in purifying our j^east by 

 successive growths, we may add to every 100 c.c. {3^ fluid 

 ounces) of wort that we employ from ten to twelve drops of 

 phenol water, containing 10 per cent, of the acid. The action 

 of the phenol, which at first is invariably combined with that 

 of the oxygen of the air, tends to destroy the vitality of many 

 of the cells sown, involving to some extent also the yeast which 

 we are interested in preserving. But amongst the number of 

 cells that are afiected those which are less abundant, that is 

 to say, those which are present as impurities, are paralj^zed 

 relatively in much greater proportion. If the acid does not 

 destroy them it greatly checks their development, and the cells 

 of yeast, which multiply continuously in vast numbers (for the 

 fermentation goes on in spite of the phenol, if this is added 

 in small quantity), gradually choke the foreign germs in a 

 succession of growths. 



By these different means, which are employed separately or 

 combined with one another, we generally manage to obtain the 

 yeast which we wish to purify in a very pure state. We need 

 scarcely add that it is always well, in the case of our purifi- 

 cations, to begin with specimens which are already as pure as it 

 is possible to obtain them. In making our choice the microscope 

 is our best guide, but it is not a sufiicient one. We should 

 be strangely deceived if we believed in the purity of a yeast for 

 the sole reason that M'hen examined under the microscope it 

 appeared to contain nothing of a foreign nature. The best 

 means of assuring ourselves of the purity of a j-east consists in 

 making some beer in one of our two-necked flasks, and leaving 



